Sizing the 12v Domestic Battery Bank

With the knowledge of the amps consumed by your 12 volt systems and appliances you can now make a reasonable assessment of the required size of your boat's 12v domestic battery bank.

In a way bank of batteries only the middleman, storing the electricity which is generated elsewhere and metering it out as required to your 12 volt systems and appliances.

A 12v domestic battery bank - but each battery should be strapped down securely

These three elements - requirement, storage and supply - must be properly matched or the whole system will be compromised.

Too little storage capacity (the batteries being of insufficient amperage), then they'll need constant recharging.

And if your charging equipment doesn't provide enough power then your batteries will never be fully charged, a situation leading to their early demise.

Getting the balance right is important on any boat, but probably more so on sailboats than powerboats because:

Boat batteries are seriously heavy; you don't want more than you need;

But conversely, your domestic battery bank must be large enough to avoid the need for excessive running of the engine-driven alternator or dedicated generator - and engine noise isn't something that we sailors readily appreciate.

The Energy Equation

Modern cruising yachts demand increasing amounts of its energy to power their ever more complex systems, so adequate means of both storing and generating it are of vital importance. Fundamentally - if disappointment is to be avoided - the relationship between supply and demand must be properly managed, ie:

Energy In = Energy Out

which means you can only take out what you put in. Rather like a well-managed deposit account - hence 'battery banks', possibly?

So to properly manage our energy equation, we need to evaluate both sides of it:

The 'Demand' side - How much energy do we require to feed our electrical systems?

The 'Supply' side - How can we generate and store enough of it to meet that requirement?

Sizing the 12v Domestic Battery Bank

It's important to match the battery bank capacity to the current requirement. Too low a capacity and the charge times will be long and frequent; too large and it will be difficult to fully charge it through an engine driven alternator.

Before we go any further, we should have a look at battery amp-hour ratings. These refer to the available current over a nominal period until a specified voltage is reached. Rates are normally specified as either a 10 hour rate or, almost invariably these days, at a 20 hour rate.

This means that a battery rated at 100Ah at a 10 hour rate with a final voltage per cell of 1.7 volts is capable of delivering 10 amps for 10 hours, when a cell voltage of 1.7 volts is attained. Thus a 6 cell 12 volt battery at this stage would show a residual voltage of 10.2 volts - or flat, not a good shape for a battery to be in.

But don't be fooled into thinking that a battery rated at 20 hours is 'bigger' than one rated at 10 hours for the same capacity. It isn't, it's the other way round. If you were to choose a 100Ah battery rated at 20 hours, it would have 10% to 15% less capacity than a 100Ah battery rated at 10 hours.

So, a battery's capacity depends on how fast you discharge it. It's very important to understand that the faster you drain your batteries, the less capacity you will have at your disposal. The good news is that the converse is also the case. Discharge your boat batteries at a slower rate than the one specified and you will find that you have more capacity than you thought you had.

Open Circuit Voltages
An open circuit is one where the battery has been allowed to stabilise after charging and all DC loads have been turned off.

If voltages are measured immediately after charging the voltages could measure 13.4v due to the presence of surface charge on the battery plates.

To remove this, apply a load of 15 to 20watts for ten minutes.

12.8v: fully charged
12.6v: 80% charged, 20% discharged
12.4v: 70% charged, 30% discharged
12.2v: 50% charged, 50% discharged
12.0v: 30% charged, 70% discharged
11.8v: completely discharged

But of course we don't want to discharge our batteries until they're flat - they won't stand too many such cycles until they're ruined. Conventional wisdom has it that they should only be discharged to 50% of their capacity.

So for a 12 hour current draw of 175A would we need a battery bank of 350Ah?

In an ideal world that would be correct, but the reality is rather different. With the alternators supplied as standard equipment on most yachts, batteries are rarely above 70% charged and can't be fully recharged with the standard regulator fitted, and the resulting sulphation on the battery banks will reduce capacity by a further 10%.

Thus in this example a battery bank of 2 x 175Ah batteries could only be considered as the absolute minimum, and only then if you've fitted a high output alternator and a 'smart' regulator to minimise the losses - and are prepared to run the engine every 12 hours.

Ideally, the current draw should be matched to the discharge characteristics of the batteries. As we know, the faster a battery is discharged above its nominal rating, the lower the real amp-hour capacity will be.

Conversely, the slower a battery is discharged below its nominal rating, the higher the real amp-hour capacity. In our example the latter case would appear to be true - a 14.6Ah draw down against a nominal discharge of 17.5Ah from each of the 175Ah batteries.

But as the batteries are rated at 10 hours the battery will discharge longer and faster during the 12 hour period of the draw down, so it would be prudent to install two boat batteries of the next size up. Two 200Ah marine batteries would be fine for the domestic battery bank.